What's the difference between 'Group by GL Date' and 'Summarize by GL Date'?

Navigation: GL-> Setup->Financials->Accounting Setup Manager -> Accounting Setups ->Subledger Accounting Options -> Update Accounting Options -> General Ledger Journal Entry Summarization

There are 5 options. Summarize by GL Date, Summarize by GL Period, Group by GL Date, Group by GL Period, No Summarization.

What is the difference between Summarize by GL Date and Group by GL Date?

Thanks,

Xiaoyi